The Islamist militant group Boko Haram has about 4,000-6,000 “hardcore” fighters, US intelligence officials have said.

In an assessment of the group, whose five-year uprising including massacres and kidnappings has spread from Nigeria into neighbouring countries, the officials said they did not believe it posed major threat to Nigeria’s oilfields.

The officials, who spoke on condition of anonymity, said the militants were believed to be still holding 300 schoolgirls they kidnapped last year and had dispersed them to multiple locations.

Meanwhile, officials in Niger said their troops killed 109 Boko Haram fighters as they repulsed attacks on Bosso and Diffa, two south-eastern towns near the Nigerian border. Four Niger soldiers were killed, 13 were wounded and two are missing after the fighting in Bosso against Boko Haram, whose five-year insurgency is spreading from Nigeria to neighbouring states. Four Niger soldiers were wounded in Diffa.

“At around 9am elements of the Boko Haram terrorist group launched two simultaneous attacks at Bosso and Diffa. At Bosso, Niger’s defence forces helped by Chadian troops neutralised the assailants,” said the statement by defence minister Karidio Mahamadou on state television.

Chad deployed war planes to repulse the attack, military officials in Niger said earlier.

There was no independent confirmation of the numbers killed. General Yaya Doud, commander of Chadian forces deployed north of Lake Chad, was shot in the stomach in Bosso, a Chadian security source said. He has been evacuated to hospital in N’Djamena for treatment.

“The Boko Haram attack from Malam Fatori (in Nigeria) against the town of Bosso and the bridge at Doutchi in the Diffa region has been repulsed. We have Chadian planes bombarding the locality,” said a Niger military source.

Boko Haram has seized territory in north-eastern Nigeria as part of a five-year insurgency for an Islamist state. Around 10,000 people were killed last year and the militants increasingly stage cross border attacks.

The insurgency is the worst threat to Nigeria’s security as the nation, Africa’s top oil producer and biggest economy, heads to a presidential election on 14 February.

The militants are also increasingly threatening neighbouring Chad, Niger and Cameroon, prompting regional leaders to come up with a joint plan to defeat them.